1. Field of the Invention
The present invention relates to an LCD panel of large cell gap tolerance and an LCD projector using it, and more particularly, to an LCD panel of large cell gap tolerance and an LCD projector using it, in which a brightness change in each gray level and a transmittance rate change due to an alignment error are small by compensating a cell gap change.
2. Background of the Related Art
Recently, an Liquid Crystal Display(LCD) projector is utilized in a large display device for an HDTV or a large display device used for an announcement conference such as a seminar because of being small in its volume and easily adjusted in its projection screen size. In general, the LCD projector includes dichroic mirrors for dividing white light outputted from a light source into red, green and blue lights colors, LCD panels for modulating the divided lights with the dichroic mirrors and a projection lens for adding and magnifying lights outputted from the LCD panels.
A conventional LCD panel used in the LCD projector includes an Liquid Crystal (LC), of which properties are changed according to input voltage, pixel electrodes and a-common electrodes for applying the input voltage to the LC, and base plates on which the electrodes are formed. Furthermore, the base plates, on which the pixel electrodes are formed, further include TFTs (Thin Film Transistors) for applying/blocking voltage to/from LC layers every pixel. Here, the TFT is most used in the LCD panel of the LCD projector as being easy in multi-gray and fast response.
However, such LCD panel shows different properties according to its thickness. That is, according to the LCD panel thickness, a cell transmittance rate is changed small in high gray level but large in low gray level. Additionally, the LCD panel brightness is gradually lowered because an aperture ratio is lowered when the resolution of the LCD panel becomes gradually high.
Even though the LCD is manufactured by highly controlled processes, the LCD thickness is not uniform to depending on the position even in the same LCD panel. Furthermore, the different LCD panel thickness causes a different thermal expansion to depending on the position (or point) in the same LCD panel because the LCD panel receives lots of infrared rays and visible rays from the light source. For example, if there is a difference of temperature of 1° C. between glass base plates of the LCD panel, the cell gap is changed about 0.1 μm.
FIG. 1 illustrates a cross-sectional view of an LCD panel on which a conventional microlens is attached.
As shown in FIG. 1, the LCD panel, on which the micro lens is attached, includes a first glass base plate 1, TFTs 2 and pixel electrodes 3 formed on the first glass base plate 1, a second glass base plate 6 formed in a predetermined interval from the first glass base plate 1, a common electrode 5 directing the TFTs 2 and the pixel electrodes 3 and formed on the second glass base plate 6, an LC layer 4 filled with LC and formed between the pixel electrodes and the common electrode 5, and a micro lens 7 attached on an opposite side of the side the second glass base plate 6, on which the common electrode 5 is attached.
The micro-lens 7 transmits light entering a BM(Black Matrix), a signal line or a scan line (, which are non-modulated areas) toward the pixel electrodes and increase effective aperture ratio.
FIG. 2 illustrates a detailed cross-sectional view of the pixel electrode of FIG. 1. A gate electrode 9 of the TFT 2 is connected to the scan line of the LCD panel, a source electrode 8 is connected to the signal line of the LCD panel, and a drain electrode 10 is connected to the pixel electrode 5 of the LCD panel.
An operation method of the LCD panel on which the micro lens is attached will be described as follows.
In case of a selection period of time:
If voltage of the gate electrode 9 connected to the scan line is larger than that of the source electrode 8 connected to the signal line, a connection resistance of a channel formed between the drain electrode 10 and the source electrode 8 becomes small. Therefore, voltage of the source electrode 8 connected to the signal line is formed between the pixel electrode 3 and the LC layer 4.
In case of a non-selection period of time:
If voltage of the gate electrode 9 connected to the scan line is smaller than that of the source electrode 8 connected to the signal line, the connection resistance of a channel formed between the drain electrode 10 and the source electrode 8 becomes larger, and thereby the drain electrode 10 and the source
electrode 8 are electrically isolated. Therefore, the LC layer 4 keeps electric charge accumulated during the selection period of time.
If root means square (rms) voltage, which is applied to the LC layer 4 formed between the pixel electrode 3 and the common electrode 5, is controlled when linearly polarized light emitted from a polarizer (not shown) mounted on the outside of the micro lens 7 passes the LC layer 4 through the micro lens 7, the polarized state of the light is changed. The LCD pixel brightness is changed by the changed light selectively passing the analyzer mounted to the outside of the first glass base plate 1 of the LCID panel, and thereby the pixel brightness change as data information.
Meanwhile, the LCD projector according to the prior arts, according to LC mode, uses a 90° TN mode in case of a transmission type, a parallel oriented ECB (Electric Controlled Birefringence) mode in case of a reflection type, or a TN mode having a twist angle less than 90°.
Recently, the LCD panel used in the LCD projector shows resolution of 0.7 inch XGA level and may show resolution of 0.5 inch XGA level in the future.
However, the conventional LCD, on which the micro lens is attached, and the LCD projector using it has still several problems that the brightness change in each gray level is large and the transmittance rate change due to alignment error is large, and thereby the video quality is deteriorated and the production yield is low. The problems will be described in more detail, taking examples as follows.
FIG. 3 illustrates a graph showing a relative transmittance change in each gray level of the LCD panel according to the cell gap of the prior art.
As shown in FIG. 3, G0 indicates the reference, so the transmittance change is zero when the thickness is 4.0 μm, G1 indicates the relative transmittance change when the cell gap is 4.4 μm, and G2 indicates the relative transmittance change when the cell gap is 3.6 μm. Therefore, because the cell relative transmittance change differs about 40% or more according to the cell gap, the brightness change in each gray level is still large.
Therefore, even though the conventional LCD panel, on which the micro-lens is attached, places the focus on the pixel electrode, the brightness change in each gray level is large and the transmittance rate change due to the alignment error is large, thereby deteriorating the video quality and lowering the production yield.
Meanwhile, the LCD projector using the conventional LCD panel, on which the micro lens is attached, also has the above problems that the brightness change in each gray level is large, the transmittance rate change due to the alignment error is large, thereby deteriorating the video quality and lowering the production yield.